Trezor can be used as either hot or cold storage. You can easily move coins to trezor's address and use it as cold storage but also allows you to plug the trezor to an internet machine and safely transfer coins as a hot wallet.

The cold pi is intended for cold storage only. Of course you can still spend coins on the Cold Pi address but the way to do it is a bit more complicated than trezor. You will need to export the watch only address from your cold pi to an internet machine, create a transaction using the watch only address, move the transaction data via USB, sign it on the cold pi and move it back to the internet machine.